"For a whole week now," says the composer-lyricist, "nobody's called to say they wanted me to adjust any music or lyrics. I'm approaching that useless place where I can just sit back and enjoy the show."

The "show" is, in fact, "Happy Days: The Musical," currently in previews, prior to opening on Sunday at the Paper Mill Playhouse in Millburn. The musical is loosely based on the iconic ABC sitcom. From 1974 to 1984, viewers relished the 1950s world of Richie, Mr. C and "The Fonz," when "Sit on it!" was as profane as expletives went.

Garry Marshall, the series' creator, called Williams to provide the score for the musical. They'd worked twice before in two of Marshall's ¤'70s sitcoms: Williams played himself in an episode of "The Odd Couple," and later he wrote the lyrics to the theme song of "The Love Boat."

"Garry and I started 'Happy Days' five years ago. Stephen Schwartz saw it after we'd been working for three years," says Williams, citing the composer of "Wicked" and "Godspell."

"When he said, 'You're in great shape for only three years,' I wanted to grab him and say, 'What do you mean for three years? I can't work on this forever. How much time do I have left?'" (For the record, Williams turned 67 last week.)

WILLIAM PERLMAN/THE STAR-LEDGERComposer Paul Williams

Williams now admits Schwartz had a point. He and Marshall soon discovered (to quote one of his own lyrics) "We've Only Just Begun," even after the show's run last year in Burbank, Calif., near Williams' home.

"We'd already gone through all sorts of plots," he says. "One, if you can believe it, even involved building a children's hospital."

The biggest challenge, Williams says, was finding a way to make Arthur "Fonzie" Fonzarelli sing. "He's such a cool guy, how do you get him to open up to his concerns and fears and sing?" he asks.

The answer was found in two words: Pinky Tuscadero. While she was a fleeting character on the TV series, she's a major player in the musical.

"Fonzie may not want to change in a world that's just about to change," says Williams, "but the ¤'60s are on the way. Can Pinky get him to say he's been wrong about certain things? Besides, a musical's got to have a love story."

Williams wasn't used to a musical's ever-evolving, lengthy process for a couple of reasons. First, he recalls that he wrote the music and lyrics for one of his biggest hits, "(Just) An Old-Fashioned Love Song," in 20 minutes. Secondly, when "Bugsy Malone," the 1976 movie for which he wrote the score, became a London stage musical, it was already written, so he wasn't much needed.

"Finally, though," he says, "after the run we just finished (at the Norma Terris Theatre) in Connecticut, we're as done as we'll ever be." The month-long engagement at the Paper Mill is the second leg of a 41-city tour. Broadway is the next goal.

The songwriter points out that "Happy Days" audiences shouldn't expect a plethora of pop songs. "I've adhered to the musical theater goal of making sure that songs advanced the action of the plot."

Still, he hopes his work will be as well-received as the ¤'70s standards for which he co-wrote both words and music.

"I've made money, sure," he says, "but there have been many wonderful moments I call 'heart payments.' That's when people come up to you and tell you how much your songs have touched them. Children of one-parent families have said their single mothers used to sing 'You and Me Against the World' to them. Couples say they used 'We've Only Just Begun' as the first dance at their weddings."

His personal favorite, though, is "The Rainbow Connection" from "The Muppet Movie." Says Williams, "Jim Henson of the Muppets was one of the two greatest collaborations I ever had. Garry Marshall is the other."

Notably absent from that short list is Barbra Streisand, with whom Williams wrote the Oscar-winning song "Evergreen" from 1976's "A Star Is Born."

"No, Barbra was great, too," he says hastily. "She called me after I'd written 'You and Me Against the World' and said she'd like a song like that at the end of 'A Star Is Born.' In my mind, though, I heard her say, 'We'd like you to write all the songs.'

"Now I know it's not what she said at all, but I came into the meeting, laid out all my ideas for songs, some titles even. That made her say, 'You're not intimidated, are you?' I said no, and wound up writing seven of the 10 songs in the film."

How can a man who's 5-foot-2 not be intimidated? "When you're my size," he says, "you learn in grammar school that you can't fight, so you'd better be funny."

He stops and shakes his head. "After you enter successful years, you enter the arrogant years. Then mix in some alcoholism and drug addiction, and that was me in the ¤'80s, after the immensely productive ¤'70s. I did 48 'Tonight Shows' and remember six.

"But last March," he says, "I celebrated 17 years of sobriety. My recovery is a huge part of my life, and it's allowed me to have many happy days."